As known, many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in containers made of sterilized sheet packaging material.
A typical example of this type of container is the parallelepiped-shaped container for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material, and a number of lamination layers of heat-sealable plastic material, e.g. polyethylene films, covering both sides of the base layer.
In the case of aseptic containers for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-sealable plastic material, and is in turn covered with another layer of heat-sealable plastic material forming the inner face of the container eventually contacting the food product.
Containers of this sort are normally produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; the web of packaging material so sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections to form pillow packs, which are then folded mechanically to form respective finished, e.g. substantially parallelepiped-shaped, containers.
Alternatively, the packaging material may be cut into blanks, which are formed into containers on forming spindles, and the containers are filled with the food product and sealed. One example of this type of container is the so-called “gable-top” container known by the trade name Tetra Rex (registered trademark).
To open the containers described above, various solutions have been proposed, including reclosable opening devices made of plastic material and substantially comprising a pouring spout, defining a through pouring opening and fitted to a hole in a wall of the container.
When producing the opening device, the opening of the pouring spout is sealed by a plastic closing element connected integrally to the pouring spout and detachable from it along a smaller-section annular tearable membrane; the closing element extends at the same level as the packaging material so as to seal the hole in the wall of the container.
A removable, e.g. screw or hinged, lid can be fitted to the pouring spout to outwardly closing the latter and allowing closure of the container after the first opening by removing the closing element.
An example of the above-mentioned opening device is shown in EP-A-2508432. In this case, the pouring spout and the closing element are injection molded in one piece directly on the hole formed in the packaging material so as to seal it.
In particular, the portion of the packaging material provided with the hole on which the pouring spout and the closing element are to be formed is placed between two molds in an open configuration. The molds are then displaced towards the packaging material to reach a closed configuration, in which they cooperate with opposite faces of the packaging material and define a closed mold cavity housing the above-mentioned hole. The injection molding operation is performed by injecting the molten plastic material in the mold cavity defined by the molds in the closed configuration. More specifically, the molten plastic material is forced to fill completely the mold cavity so as to form the pouring spout and the closing element.
Subsequently, a lid is fitted to the pouring spout to close the pouring opening at a region thereof different from that closed by the closing element.
In particular, in the position closing the pouring spout, the lid has a lowered end wall inserted within the pouring spout and placed in contact with the closing element. The lid also has a conductive layer in its bottom surface contacting the closing element.
The lid is then welded to the closing element by an induction heating welding operation. In greater details, an electric current is induced in the conductive layer of the lid so as to generate a localized heat adapted to melt the plastic materials of the contacting surfaces which then solidify in a welded condition.
It is also specified that the conductive layer may be provided in the closing element instead of the lid.
The described solution of opening device permits a one-step, low-effort opening of the container by simply raising the lid. During this movement, a pulling action is exerted on the closing element secured to the lid, which is torn from the pouring spout along the smaller-section annular membrane. In particular, the tearing action starts at a given point of the annular membrane and then propagates to the rest of the membrane up to produce a complete detachment of the closing element from the pouring spout.
However, the solution disclosed in EP-A-2508432 has the drawback that the induction heating welding operation is performed too close to the packaging material with some concerns as to the integrity of the latter. In particular, in order to avoid any possible damage to the packaging material in the zone adjacent to the opening device, it is necessary to control and to limit the pressure and the temperature reached during the welding operation. As a matter of fact, the electromagnetic energy generated by the induction process is partially transferred to the packaging material in the form of heat, which may produce damages to the packaging material itself.
Due to the temperature and pressure limitations, the time necessary to perform the welding operation is relatively high and cannot be reduced without increasing the risks of producing possible damages to the packaging material or reducing the quality of the welding.